The present invention relates to a battery alarm system for monitoring output voltage of a battery used for a portable telephone, etc., and producing an alarm on recognizing the reduction in the output voltage.
A portable telephone requires an alarm system for producing an alarm (sound or indicator) to notify the user of the reduction in the output voltage of a battery which causes an inoperative condition of the portable telephone. In response to the alarm, the user determines whether he will stop using the portable telephone or take an appropriate action such as charging the battery. A precise and stable alarm should be produced to make a correct determination.
FIG. 1 shows a configuration of a conventional battery alarm system.
In FIG. 1, a voltage detector IC 32 and a system power source 35 are connected to the output terminal of a battery 31. The output of the voltage detector IC 32 is applied to a CPU 33. The CPU 33 and a program storage 34 are provided with electric power by the system power source 35, and the CPU 33 performs a predetermined process stored in the program storage 34.
In the above described configuration, the voltage detector IC 32 detects an output voltage of the battery 31, compares the obtained value with a predetermined threshold Vth, and notifies the CPU 33 of the result. The CPU 33 determines the alarm state when it recognizes that the output voltage of the battery 31 is lower than the threshold Vth. Then, it causes an alarm to be sounded with a buzzer 36 and a blinking alarm, etc. to be made on a display device.
In a conventional battery alarm system, however, the generation of an alarm greatly depends on the capability of a voltage detector IC 32. That is, the detector notifies a CPU 33 of information of "the alarm state" and "the normal state" changeably within a short time when the output voltage of the battery 31 is unstable and fluctuating, thus generating an alarm unstably.
Besides, in a conventional system, a threshold Vth of the voltage detector IC 32 is fixed to a determined value. Therefore, when an alarm generating value of the voltage must be modified, another voltage detector IC having another threshold should replace the first detector, thus the system becomes inoperative during an emergency modification.